1. Field of the Invention
The present invention relates to a two-phase stainless steel, a method of manufacturing the same, a diaphragm using the two-phase stainless steel, a pressure sensor including the diaphragm, and a diaphragm valve including the diaphragm.
2. Description of the Related Art
A metal diaphragm is a portion of a pressure sensor that is in contact with liquid and is made of an alloy material having excellent corrosion resistance and excellent pressure resistance. Since the metal diaphragm is actually used in a variety of environments, the material thereof needs to be determined in view of the use environments including the liquid properties, pressure, temperature and the like of a process fluid.
Conventionally used materials of the metal diaphragm include a Co based alloy, a Ni based alloy, and a precipitation-hardened stainless steel.
For example, a known diaphragm is made of a metal material provided by performing heat treatment on a solid solution of metal material including a Fe—Ni based alloy or a Fe—Ni—Co based alloy mixed with a precipitation-strengthened element such as Ti, Al, or Nb and performing aging-effect treatment to increase the strength (see JP-A-2000-275128 (Patent Literature 1)).
A known metallic diaphragm in a pressure sensor is made of an alloy composed mainly of 20 to 40% Cr+Mo, 20 to 50% Ni, and 25 to 45% Co, and is provided by performing cold working of 20% or higher and then heat treatment at 400 to 600° C. (see JP-A-5-013782 (Patent Literature 2)).
A disclosed method of manufacturing a thin-film sensor includes, in a thin-film pressure sensor using a metal diaphragm, performing the precipitation-strengthening of the metal diaphragm and the step of forming the thin-film pressure sensor at the same time (see JP-A-1-173846 (Patent Literature 3)).
A known pressure detector is configured by bonding a plate glass to a diaphragm face of a pressure-receiving metal diaphragm made of a Kovar material through a low-melting glass layer, placing a strain gauge semiconductor chip on the plate glass, and anodic-bonding the plate glass to the semiconductor chip (see JP-A-62-291533 (Patent Literature 4)).
A known semiconductor pressure sensor is provided by forming a fixed header from a Fe—Ni based alloy having a composition of 36 to 40% Ni by weight and the balance Fe, providing an external pressure introducing tube in a central portion of the fixed header, attaching a stem body, and including a semiconductor pressure sensor element mounted on the fixed header (see JP-A-58-148437 (Patent Literature 5)).
The alloys forming the metal diaphragm in the related arts contain Cr added thereto, and a passivation film made of a dense chromium oxide layer is formed on the surface of the alloy to have excellent corrosion resistance. Although a Ti alloy may be used as the material of the metal diaphragm, the Ti alloy provides excellent corrosion resistance since Ti having a high affinity for oxygen forms a titanium oxide layer on the surface.
A mechanical property required of the metal diaphragm is a high proof stress. The principle of the pressure sensor using the metal diaphragm is based on electrically sensing the amount of deformation of the metal diaphragm through a strain gauge provided at the metal diaphragm when a force is applied by a process fluid. Thus, the reproducibility of the pressure measurement is ensured when the metal diaphragm is elastically deformed. If a stress more than the proof stress is applied by the process fluid, the metal diaphragm is plastically deformed and cannot show a proper pressure value after the plastic deformation. To maintain the accurate pressure sensing performance, the metal diaphragm needs to have a proof stress higher than the stress applied by the process fluid.
To add the strain detecting function in the metal diaphragm, the following two structures are generally employed. The first structure includes a strain gauge bonded to the surface of the metal diaphragm opposite to the liquid contact surface, and the second structure involves using the metal diaphragm itself as a strain element. Both structures require the smoothing of the surface of the metal diaphragm in order to achieve favorable accuracy in strain sensing. For this reason, the surface of the diaphragm is finished into a smooth surface by performing various polishing steps thereon.
Thus, it is important to evaluate and select an advantageous material for the pressure sensor with excellent corrosion resistance and excellent pressure resistance in view of the use environments and the manufacture considerations in assembly of the pressure sensor.
Many of alloys containing a sufficient amount of Cr exhibit excellent corrosion resistance since a dense passivation film of chromium oxide is formed under a certain oxidizing environment. However, under a non-oxidizing environment in which a passivation film is hardly formed or under a strongly oxidizing environment in which a passivation film is further oxidized, the passivation film is broken to expose and dissolve an underlying layer. Examples of such a non-oxidizing environment include an alkaline solution and a neutral solution at high temperature. The strongly oxidizing environment may include the use of an electrochemical anti-corrosive technique to cause interference which results in unintentional application of an anode potential. The metal diaphragm may be placed in any of these environments, and to offer the material suitable for these environments is a technological challenge.
The proof stress of the Co—Ni based alloy conventionally used widely as the material of the metal diaphragm can be increased through some treatment to approximately 1500 to 1600 MPa to achieve a higher strength. However, the same mechanical property cannot be provided by another alloy, for example, an austenitic stainless steel, a ferrite stainless steel, or a Ti alloy.
For the metal diaphragm made of the precipitation-hardened alloy, the polishing of the surface of the metal diaphragm involves predominant polishing of a soft base phase to cause the protrusion or removal of a hard phase, thereby preventing the production of a favorable smooth condition. This presents the problem in which the orderly pattern of the strain gauge formed on the surface of the metal diaphragm is lost to reduce the accuracy in pressure sensing. In addition, for the metal diaphragm made of the precipitation-hardened alloy, a precipitated phase may be removed under a corrosive environment to form a pit, and the pit may serve as a starting point which may grow into a break.
The Ti alloy forming the metal diaphragm is prone to flaws and is damaged due to a slight contact after the polishing finish, so that there is a need to develop a better material for the metal diaphragm.
The present invention has been made in view of the situations in the related art, and it is an object thereof to provide a two-phase stainless steel, a method of manufacturing the same, a metal diaphragm using the two-phase stainless steel, and a pressure sensor including the diaphragm, capable of achieving a higher strength, excellent corrosion resistance, and a smooth surface condition. It is another object of the present invention to provide a diaphragm valve including the diaphragm.